"How many cars move beside the river?"
Translation:Hány autó halad a folyó mellett?
Basically, "uncountable versus countable." Usually, hány is translated as "how many" and is asking about something you can answer by counting (discrete units). Mennyi is "how much" and is asking about a quantity that is not necessarily countable.
How many apples are in the basket? Hány alma van a kosárban?
How much water is in the tub? Mennyi víz van a kádban?
But Hungarian is more forgiving here, you can use "mennyi" with countable objects, as well. Not the other way around, though. Let's say it depends on whether you expect a "countable answer".
Q: Mennyi alma? - A: Két kiló (Two kilos). Or you can say the number of apples.
Q: Hány alma? - A: Tizenöt (15). I was definitely asking the number of apples.
And "Mennyi" can also be used in exclamations like "Wow, look at all those cars!" - "Mennyi autó!" In this case, the countability does not matter at all. You use "mennyi".